Extrusion of thermoplastic materials, such as polyvinyl chloride (PVC) resin, generally requires heating an extrudable mixture to a temperature at which it can undergo plastic flow and subsequently forcing it through a plate containing one or more orifice openings (sometimes referred to as a die), to impart a profile shape to the emerging extrudate. The extrudate is then cooled until it becomes sufficiently rigid to retaining the profile shape.
In general an extrusion feedstock composition is prepared by combining a thermoplastic polymer, such as PVC resin, with one or more adjuvants, such as lubricants. For example, extrudable PVC compositions are described in Handbook of Plastic Materials and Technology, Ed. I. Rubin, Wiley-Interscience, John Wiley & Sons, Inc. New York, 1990 and Plastics Additives and Modifiers Handbook, Ed. J. Edenbaum, Van Nostrand Reinhold, New York 1992, Chapter 3, each of which is incorporated herein by reference. Examples of additional components include filler, for example calcium carbonate, processing aids, for example acrylic copolymers, and stabilizer which imparts heat stability to the composition during the extrusion process, for example, a tin based stabilizer such as TM281® from Rohm & Haas. The composition is commonly then “fused,” that is, subjected to heating and blending until it forms a plastic composition which is substantially homogeneous. The various additives and the amounts of the additives used influence the properties of the extrudable composition both during the fusion process and after it has been “fused.” The properties of the composition in turn affect the performance, such as processability, of the material during extrusion or in follow-on processes in which the extruded material is employed. Of the various components comprising typical extrudable PVC compositions, the lubricant included in the composition can have an important influence on one or more of the properties related to processability, as well as on the quality of an extruded article. Standards for the properties and performance of extruded thermoplastic articles are well known, for example, ASTM Standard D3679-03e2 for PVC siding and D1785-03 for PVC pipe, as well as DIN Standard 16830-3 for high-impact PVC profiles for windows and doors.
Dimensional stability is an important characteristic affecting the commercial value of an extruded product and hence of the processes and the components used to make the extruded product. Dimensional stability is assessed by observing the amount by which an extruded shape expands during solidification after it emerges from the die of an extrusion apparatus. The amount of expansion is sometimes referred to as die swell.
For a given extrudable composition, it is generally possible to increase the rate at which material is extruded. However, such extrusion rate increases have practical limitations. For example, it is possible to obtain extrusion rate increases by operating the extruder at a higher temperature and/or at a higher head pressure. However, at some point of increasing extruder temperature the extrudable composition will generally begin to scorch, and the surface and structural qualities of the extrudate will begin to degrade. Furthermore, as the head pressure or extruder torque is increased, a point is generally reached at which the dimensional stability and/or surface condition of the extruded material is unacceptable. These process limitations have created practical limits on the rate at which prior compositions could be extruded.
Another mechanism that is at least theoretically available to achieve higher extrusion rates is to increase the amount and/or type of lubricant incorporated into the extrudable composition. However, it is also possible that increasing the amount of the lubricant or changing the type of lubricant can have deleterious effects on the extruded product and/or on other aspects of the extrusion process. For example, increasing the amounts of certain lubricants known as internal lubricant may compromise one or more of the inherent physical properties of an extruded article, such as a reduction in the tensile strength of an extruded article. Also, increased amounts of external lubricant may negatively impact not only the strength of the extrudate but may also increase the fusion time, which increases the overall processing time.
Applicants have thus come to recognize the need for extrudable compositions, particularly extrudable PVC-based compositions, having one or more of those desirable extrusion characteristics associated with high levels of lubricant while reducing or substantially eliminating one or more or the adverse effects that would otherwise be associated with a high level of lubricant. The present methods and extrudable compositions meet this and other needs.